But despite the rebound in migrants attempting to enter the country illegally, the overall number detained along the southwestern border still hit its lowest mark on record in fiscal year 2017, according to U.S. Customs and Border Protection.

Border Patrol agents apprehended 303,916 border crossers for the 2017 fiscal year, which ended in September. At the same time, customs officers at ports of entry processed 111,275 inadmissible individuals — migrants who were denied legal entry into the country. That includes Central American minors and families seeking refuge in the U.S.

In another milestone, a majority, some 58%, of the migrants apprehended at the border were from countries other than Mexico — mostly from Central America. That's the largest share ever, and only the second year that Mexican migrants were less than half of the total apprehensions.

A 1-year-old from El Salvador clings to his mother after she turned themselves in to U.S. Border Patrol agents. They are part of a growing group of migrants to the U.S. who are fleeing gang violence in Central America.(Photo: John Moore, Getty Images)

'Trump effect' quickly fades

President Trump's arrival in the White House in January 2017 appeared to trigger the historic dip in apprehensions for the first four months of his presidency.

The number of apprehensions, the best measure of attempted illegal immigration, reached in April its lowest point since Customs and Border Protection began tracking that information.

At the time, the Trump administration took credit for the decrease in apprehensions and people turning themselves in at the border.

But migrant advocates predicted the decline would be temporary and that many migrants were waiting to see what would happen under Trump before attempting to enter the U.S. illegally. They anticipated the number entering the country illegally would increase once the novelty of a new administration wore off.

The numbers released this week appear to show that's what happened.

With the approach of summer, Border Patrol apprehensions began to rise again. Meanwhile, customs officers at the ports of entry recorded a spike in the number of underage migrants and families from Central America turning themselves in and asking for asylum. A family is considered to be at least one adult and a minor.

The number of families that turned themselves in soared to the highest levels since the surge in Central American migration began in 2013.

“We have seen historic low numbers this year, an almost 30 percent decline in apprehensions in (fiscal year 2017),” CBP Acting Commissioner Ron Vitiello said in a statement. “But we are very concerned about the later month increases of unaccompanied minors and minors with a family member.”

ICE arrests on the rise

While fewer people crossed the border illegally, the federal government stepped up enforcement away from the border. As a result, the number of unauthorized immigrants arrested to be processed for deportation rose sharply after Trump took office.

That increase is tied to his January executive order that widened immigration enforcement and included plans to hire 10,000 more officers at Immigration and Customs Enforcement over the next four years.

Since Trump’s inauguration, on Jan. 20, arrests of unauthorized immigrants in the country’s interior rose 40% compared with the same period the year before, according to ICE.

“We need to confront and address misguided policies and loopholes that only serve as a pull factor for illegal immigration,” he said. “Finally, we need to find a solution to the dangerous sanctuary city policies and the politicians who needlessly risk innocent lives to protect criminals who are illegally present in the United States.”

Notably, the number of arrests of undocumented immigrants without criminal records more than doubled compared with the previous year, as the Trump administration expanded its enforcement priorities.

In fiscal year 2017, ICE arrested 37,734 people who had never been convicted or charged with a crime. In 2016, the number of non-criminal arrests was half that, at 15,500.